Using light and shadow in a drawing be it using traditional or digital methods can be challenging, but with a little bit of knowledge this can be made much easier. Below I will show you the basics that will help add an appearance of 3D to your drawings.
We will be using the sphere above to demonstrate the how light and shadow is affected by the shape of an object. To keep things simple I have not added any objects that could project reflected light onto the sphere as I am leaving that subject for part 2. You should be able to see the difference between the light side of the object and the shadow side. To start with I will work from the light side to the shadow side and point out what you should take note of.
First we must identify the direction in which the light is coming from which I have indicated using a yellow arrow. In your drawings you will want to find a light direction fairly soon after if not before starting and stick to it as it will dictate how your shadows will fall on the object you are drawing.
Indicated by the blue circle is the location of the highlight this is the most lit part of the object and will be in direct light. It is represented as a bright spot or usually a white spot, you can often see it in peoples eyes when they are in a well lit area. One thing that you should note is that not all objects will have a highlight, You will mostly find them on glossy surfaces, non-glossy surfaces do not have a highlight but a slightly brighter area.
Between the blue circle indicating the highlight and the other blue line in the half-tone this is the half of the object that is in light but not enough to produce a highlight. As the face of the object turns away from the light source (as you move towards the shadow side of the object) the face of the object receives less light, this causes it to slowly become darker at a gradual rate.
Between the blue line is what is called the core shadow, this is the first point where the face of the object has turned away from the light. the core shadow is the darkest part of the shadow area and will form a band around or across the object.
Within the blue lines is the rest of the shadow area in this case as there is nothing being reflected it is as dark as the core shadow. In part 2 I will explain how this area is effected by light reflected off other objects.